In
past articles we have discussed brake problems such as judder and warped
discs. We also discussed how to correct such problems. However, we did
not discuss what to do if you are simply not happy with the way your
stock braking system is functioning. So what do you do if your American
dream has all the get up and go of an F1 racing car, but the stopping
power of a toasted cheese sandwich. Well to answer this we need to look
at a problem we have not yet touched on when dealing with braking systems,
brake fade. This is most commonly seen on earlier vehicles that are
fitted with drum brakes all round. Most people get around drum brake
fade by fitting a disc brake conversion to the front of the vehicle.
However, with engines becoming more powerful the problem of brake fade
in disc brake systems is on the increase.

Commonly there are three types of brake fade, pad fade, green fade and
fluid fade.Pad fade can occur for several reasons; the most common is
over heating. The material that brake pads are made from has what is
known as a coefficient of friction curve over temperature. That is to
say the amount of grip the pad has changes at different temperatures.
All friction materials have a temperature where they are at their most
grippy. This is known as the optimum friction coefficient. There are
times that may find you using the brakes so hard that the temperature
rises above this optimum working temperature, it is at this point that
pad fade may begin to occur. To put it simply the pads become less grippy.
The results of this over heating are of often quite varied. The most
common is the glazed pad. This is where the pad reaches a high enough
temperature for the bonding material of the pad to melt and smear across
the surface of the friction material. When this bonding material cools
it causes a glaze on the surface of the pad that covers the friction
material, thus reducing the efficiency of the pads. If the temperature
keeps rising even the friction material itself will begin to melt and
possibly vaporize. When this occurs the pad rides above the disc on
a layer of vaporized metal and friction material. This layer then acts
just like a lubricant again reducing braking efficiency to almost nothing.
Brake fade will usually build up over time and you are able to compensate
for it. However, there are some friction materials in which the coefficient
of friction drops off suddenly when a certain temperature is reached,
in extreme situations this can result in dangerous and sudden brake
fade.

Green
fade is most commonly associated with new pads. The friction material
of brake pads is usually held together with a phenolic resin binder.
These binders are thermosetting plastic resins that have a high resistance
to heat. When new brake pads are fitted and used for the first time,
these resins will expel gas or cure if used hard during the initial
bedding in period. Once again this can cause a cushion of gas between
the new pad and the disc. As with pad fade this will obviously reduce
braking efficiency. The real danger lies in the fact that many people
will fit new pads and assume their braking system is back to normal.
Correctly bedding the pads in is the easiest solution to this problem.
Again the results of this type of fade is a glazed pad. The pads in
fig.1 had only been fitted to the vehicle for two or three
days, but in that time it has become glazed. As a comparison fig.2
shows a pad in new condition. Notice the literal shiny glazed appearance
of the pad in fig.1, this is typical of a pad that
has been exposed to temperatures higher than the optimum operating temperature.
In this case the resulting glaze is probably due to green fade.

Fig 1. Pads have become glazed

Fig 2. New sinted apperence

As mention earlier,
fade commonly occurs in drum brake systems. The glazed condition of the
brake shoe shown in fig.3 is the result of excessive
heat in a drum brake system; a new shoe is shown in fig.4
for comparison. The system this shoe was part of would almost certainly
of suffered brake fade. Again these shoes had only been fitted to the
vehicle for two to three days, so this condition is probably also due
to green fade. On closer inspection of fig.3 you may
notice some lighter coloured bands running across the shoe. This is damage
caused to the surface of the shoe lining by a drum with imperfection in
its braking surface. The chances are that the drum probably requires changing.
Similar damage can occur to brake pads when a brake disc needs replacing.

Fig 3. Used and glazed brake shoes

Fig 4. New brake shoes

Moving on to fluid fade, this is caused
by the brake fluid overheating and actually boiling in the caliper.
This boiling produces gas bubbles in the brake lines. Because air is
compressible the brake pedal will become spongy. In a worst case scenario
the pedal will become so spongy that it will travel to the floor with
very little effect on braking. You’ll be pleased to know that
there are several simple ways to improve your braking system and cure
any brake fade problems that you may have. The obvious thing we have
to do is get heat away from the brakes quickly and efficiently. The
first thing to do here will be to upgrade the vehicles brake pads. Pads
can be divided, roughly, into four groups.
Firstly we have the organic pads, fig.5. These are
what we would call an old school pad; the sort of thing cars and trucks
from the 60’s and early 1970’s would have been fitted with
out of the factory, or on the rear of some later vehicles. They are
often made of a ground up cellulose material. This is held together
with a heat resistant phenolic binder. There is usually another material
added, known as the filler, to improve the pads high temperature properties.
Up to the early 1970’s this material would have been asbestos,
but since this has been found to be carcinogenic, materials such as
kevlar and fiberglass are now used as so-called fillers. Organic pads
have a really good friction coefficient for light pedal use and work
very well at low temperatures, giving a very good brake pedal feel.
These are not recommended for any performance applications, they wear
quickly, oxidize, crumble and are almost guaranteed to cause brake fade
at high temperatures. These pads are easily identified by their light
brown or tan colour.
Next we have the group of pads known as semi-metallic, fig.6.
These pads have powdered metal added, this helps to stabilise the friction
coefficient at higher temperatures. The metal that is added is usually
brass, iron or bronze, some manufacturers will also add chopped brass
or bronze wire just to give the pad more strength. The stock pads fitted
to most new cars are usually of a semi-metallic compound. These are
a good all-round pad and can be anything from light tan with metal flecks
in to dark gray in colour. The darker colour will normally indicate
a high metal content in the pad. A higher metal content will mean that
the pad has a better friction coefficient at higher temperatures, but
will cause more wear to the brake discs than an equivalent organic pad.

Fig 5. Oganic brake pads

Fig 6. Semi-metallic brake pads

Thirdly
there are the full metallic pads, fig.7. These are
made of a sintered metal using very little binder. They may be made
from brass, bronze or copper or a combination of these. At the extreme
end of the full metallic market are the iron pads, these are for use
in high temperature situations. These pads are aimed more at the race
market and have very poor grip in the wet and the cold. They also wear
rotors out quickly. These pads generate large quantities of dark brake
dust, so you if you’re using these, be prepared to clean your
wheels regularly. The normal colour for these is a dark gray to black,
some even have a slight copper sheen to them. It should be noted that
some racing full metallics have ceramic powder add to the mix, this
again will increase the friction coefficient of the compound.Finally
we have the carbon pads, fig.8. Full on race carbon
pads are not the sort of thing that many people would even consider
for street use. The carbon pads that are designed for performance street
use are usually a semi-metallic pad that have carbon powder added to
the mix, again this improve the high temperature qualities of the pad.
Carbon pads tend to have a good wide effective heat range, working well
from cold through to high temperatures almost equally well. The pads
also create a lot of brake dust, but they are a little easier as far
as wear on the brake discs is concerned. They are a flat dark gray in
colour.

Fig 7. Full metallic pads

Fig 8. Carbon brake pads

Moving away
from pads, we could also consider using performance brake discs to help
remove heat from the system and reduce the risk of fade. There are a few
different types of disc that can be used. When brake discs were first
introduced, most were simply a solid disc. It was common to try and upgrade
to a ventilated disc. As time has gone by ventilated discs are now fitted
as a stock item to most vehicles. These discs have vents radiating out
between the inner and outer braking surfaces. They could almost be thought
of as air fans. Cool air is drawn from an area around the center of the
disc. This air then flows up the vents, because of the large amount of
surface area in the vents the air very effectively dissipates the heat
that has built up in the disc. The vents can be clearly seen on the disc
in fig.9.

Fig 9. Vented disc

Fig 10. Slotted directional disc

The
next step up from these is the slotted disc. As well as being ventilated
these also have grooves radiating from the center to the edge of the
brake surface. The grooves help to disperse the gases that lead to brake
fade, as well as help to evenly cool the braking surface. This even
cooling helps to reduce the chance of hot spots on the brake disc, and
thus reduces the chance of having the discs warp. The slotted disc shown
in fig.10 is not only slotted but also cadmium plated.
So this disc should not only brake well but should also stay looking
good. Top of the line street performance discs as well as having slots
will also be either cross-drilled or have blind drilled holes on the
braking surface. Cross-drilled discs have holes going completely through
their surface, where as blind drilled holes only go part way through
the disc. These holes help to expel even more of the gases that cause
brake fade and reduce the temperature of the disc even more than slots.
Whilst the cross-drilled discs do look extremely good, under very high
temperature conditions they should be avoided. The reason for this is
that at very high temperatures it is possible for cracks to start radiating
from these holes. Where as discs that have blind drilled holes disperse
the gases but do not crack, because the holes do not go all the way
through the disc. Many of these slotted and drilled discs are actually
designed to be direction specific. In other words there are specific
left and right hand discs for a vehicle application, even if the vehicles
stock discs are not left or right handed. Installing these discs on
the wrong side of the vehicle can cause a dramatic reduction in their
cooling ability and can reduce pad and disc life by as much as 75% of
what would normally be expected. The disc shown in fig.11
is being fitted to a C5 Corvette, this is a directional, slotted and
drilled disc, and it doesn’t get much better than this.

Fig 11.----

If
the discs mentioned above are not up to solving your braking problems,
you may need to think about upgrading the size of your brake discs.
This does, however, lead us to a whole new set of problems. Whilst larger
discs will help keep the brakes cooler, you may have to remount the
calipers or use non-stock calipers. Also if the discs are considerably
larger than stock they may not fit inside your wheels and so these will
have to be replaced as well. Keep in mind this will also mean new tyres.

Finally
a word on fluid fade. If after all of this you find you are suffering
from fluid fade, don’t despair, you have some options. Regularly
changing your brake fluid every three to four years is a good idea.
Newer fluid is less likely to boil than old. Of course your fluid may
require changing more often if you have a particularly high performance
vehicle. Alternatively you could switch to using a high grade racing
type fluid; these have a higher boiling point than regular fluids. You
could also try changing your pads before the friction material gets
exceptionally thin. The idea here is that the extra thickness of material
will act as insulation between the pad and the caliper. Finally there
are some companies that supply a thermal barrier spray that can be applied
to the back of the pad for extra insulation, although we wouldn’t
like to say just how effective these actually are.

It should be remembered
that overheating in a braking system does not always mean that the
system is not up to scratch. There is always the chance that there
are other contributing factors leading to the over heating problems.
It is possible that there is a partial brake caliper failure and that
caliper is binding on to the disc. Obviously a similar binding could
also take place in the wheel cylinders of a drum brake system. This
binding is particularly common in systems with a floating caliper.
Another item to check is the brake hose. It has been mentioned in
our other articles that these can collapse internally and cause both
caliper and drum brake systems to bind.

-------------------Fig12.

So
remember that whether you are fitting stock or performance pads or shoes
it is always worth checking the actual braking surface of your discs
or drums. You should look for any sign of grooves or what we would call
‘tram lines’, if present your new pads or shoes will wear
out extremely fast. A worst case scenario will have the friction material
cracking because of uneven pressure. Also check the braking surface
for any signs of discolouring, what is known as bluing, or cracks. If
either is present it is a sure sign that the disc or drum has been exposed
to some very high temperatures. Cracking and discolouration can also
be a sign of a very old disc or drum, either way you should seriously
consider immediate replacement. The disc in fig.12 shows
signs of both discolouration and tram lines.

You have no excuses
now. If your brakes are not working the way you want them to, hopefully
this will have pointed you in the right direction. Try a brake pad upgrade
if that’s not enough you could try discs as well. If you are driving
a vehicle that still has drums all round you might even consider a disc
brake conversion. Front initially and possibly converting the rear as
well at a later date. Finally, if after reading this you’re still
not sure which way to go, ask. Ask for advice from your local club or
parts supplier. The chances are they will be able to help, or at least
know someone who can. So as far as stopping is concerned, now could
be the time to get going with that brake upgrade. Just remember when
you’re traveling at warp factor 8 Scotty is no longer going to
be there to help slow you down.